JP3476061B2 - Water vapor partial pressure measurement method and water vapor partial pressure measurement sensor - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a water vapor partial pressure measuring technique.

[0002]

2. Description of the Related Art Limiting current type sensors are mainly used in the field of measuring oxygen concentration in gas. This product does not require a reference gas (gas with a known oxygen concentration), but its output (current value) is almost proportional to the oxygen concentration, has excellent responsiveness, and can withstand high temperatures. Applied development is actively carried out. In such a limiting current type sensor, it is known that by adjusting the voltage applied to the solid electrolyte that allows oxygen ions to permeate, it is possible to have sensitivity not only to oxygen in the gas but also to water vapor. It was That is, when a relatively high voltage is applied as a monitoring voltage to the limiting current type sensor in an atmosphere containing a large amount of water vapor such as combustion exhaust gas, the water vapor is decomposed and reduced as in the following chemical formula (I) to be purified. Output increases due to oxygen gas.

[0003]

[Chemical formula 1] 2H ₂ O → 2H ₂ + O ₂ (I)

Since the increased output corresponds to the water vapor concentration, the application electrode is switched to a relatively low voltage at which the decomposition of water vapor does not occur and a relatively high voltage at which the decomposition of water vapor occurs, or two pairs of electrodes are used. The water vapor partial pressure can be measured by providing high and low voltages respectively provided.
However, since the above-mentioned measurement detects the difference in the limiting current value when oxygen gas exists in the measured atmosphere, when oxygen does not exist in the measured atmosphere, or when the measured atmosphere is a reducing atmosphere. Cannot be measured.

[0005]

SUMMARY OF THE INVENTION The present invention provides a water vapor partial pressure measuring method capable of measuring the water vapor partial pressure in an oxygen-free atmosphere and a reducing atmosphere by using a limiting current sensor. With the goal.

[0006]

In order to solve the above-mentioned problems, a method for measuring a partial pressure of water vapor according to the present invention is a method for measuring a partial pressure of water vapor using a limiting current type sensor element as set forth in claim 1, wherein The configuration is such that the atmosphere is judged and the voltage applied to the sensor element electrode is adjusted. Also,
As described in claim 3, the water vapor partial pressure measuring sensor of the present invention is a water vapor partial pressure measuring sensor having a pair of electrodes and using a limiting current type sensor element for applying a monitoring voltage between the electrodes. From the applied voltage control means for changing the voltage applied between the electrodes, the current measuring means for detecting the current flowing between the electrodes, the current value measured by the current measuring means and the voltage applied between the electrodes Water vapor content having an atmosphere determining means for determining the atmosphere to be measured and a water vapor partial pressure calculating means for calculating a water vapor partial pressure from the current value detected by the current measuring means by a calibration curve corresponding to the atmosphere detected by the atmosphere determining means It is a pressure measurement sensor.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION An example of a limiting current type sensor element used in the present invention will be described with reference to FIG. This is a limiting current type sensor element which uses a porous substrate for controlling the diffusion of gas and has a pair of electrodes and applies a monitoring voltage between these electrodes. A zirconia solid electrolyte is sandwiched between a porous cathode and an anode made of platinum, and a porous alumina substrate is provided so as to cover the anode side. There are platinum heaters on many sides of the porous alumina substrate, and the temperature of the zirconia solid electrolyte is set to a temperature suitable for oxygen ion conduction (600
The temperature is maintained at ~ 700 ° C). Lead wires are connected to each of the electrodes and the heater so that they can be easily electrically connected to an external power source or current measuring means.

When a voltage is applied to the solid electrolyte, gas molecules having oxygen atoms existing on the anode side are decomposed, and the generated oxygen ions are conducted through the solid electrolyte and are carried to the cathode side, so that oxygen gas is generated at the cathode. Form. Thus, the gas molecules having oxygen atoms on the anode side gradually decrease, but here, they are supplied from the outside air (gas to be measured) through the pores of the alumina porous substrate. The supply at this time is diffusion-controlled, and its amount depends on the concentration of gas molecules having oxygen atoms in the measured gas.Therefore, by measuring the current flowing through the system, the concentration of gas molecules having oxygen atoms can be determined. I can know.

Here, the oxygen gas concentration is 21% or 4%
And a balance of nitrogen gas, two oxidizing atmospheres (shown as 21% O ₂ / N ₂ and 4% O ₂ / N _{2 respectively} ), a nitrogen gas atmosphere (shown as N ₂ Pure), and hydrogen gas 1 % And the balance of nitrogen gas, the limiting current type sensor element is installed in a reducing atmosphere (shown as 1% H ₂ / N ₂ ), and the applied voltage is from about −0.6 V to 0. FIG. 2 shows the result of examining the change in the current value when the voltage was swept up to around 0.6V. Here, when a positive voltage is applied in an oxidizing atmosphere, that is, when a voltage is applied in the same direction as in normal measurement, a limiting current value corresponding to the oxygen concentration is obtained in the first quadrant. At this time, the current value is almost 0 in the reducing atmosphere and does not change. Although a small amount of current flows in nitrogen gas, that is, in an atmosphere that is neither oxidizing nor reducing, the value thereof is lower than the current value in an oxidizing atmosphere.

On the other hand, when the applied voltage is negative, the current value decreases as the voltage decreases in an oxidizing atmosphere, and the limiting current characteristic according to the concentration of reducing gas (hydrogen, hydrocarbon, etc.) in a reducing atmosphere. Can be seen in the third quadrant.
However, the current is slightly zero in nitrogen gas, that is, in an atmosphere that is neither oxidizing nor reducing. Utilizing this property, the atmosphere to be measured can be determined, and the partial pressure of water vapor in the atmosphere to be measured can be measured by adjusting the voltage applied to the sensor element electrode according to the atmosphere.

[0011]

EXAMPLES The method for measuring the partial pressure of water vapor according to the present invention will be described below with reference to specific examples. The following studies were conducted using the limiting current type sensor shown in FIG. [Judgment regarding atmosphere] As described above with reference to FIG.
The current value at the time when the applied voltage to the limiting current type sensor element in the atmosphere to be measured is 0.5 V (the voltage normally applied when measuring the oxygen concentration in the oxidizing atmosphere is positive) is the current value in the nitrogen atmosphere. When it is larger, the measured atmosphere is judged to be an oxidizing atmosphere, and when it is smaller than the current value in the nitrogen atmosphere, it is judged to be a reducing atmosphere.

[Measurement of Water Vapor Partial Pressure in Oxidizing Atmosphere] FIG. 3 shows the relationship between the applied voltage and the output (current value) with respect to the water vapor concentration in an oxygen-nitrogen mixed gas having an oxygen concentration of 4%. As shown in FIG. 3, there is an inflection point near the applied voltage of 0.8 V, and when corrected by the output current value in a dry atmosphere, when the applied voltage is 0.8 V or more, particularly 1 V or more,
It can be seen that an output current value (limit current value) well corresponding to the water vapor concentration is obtained, and the water vapor partial pressure can be measured by this output current value. [Measurement of Water Vapor Partial Pressure in Reducing Atmosphere] Next, influences of applied electric power and output (current value) on water vapor concentration in hydrogen gas containing 1% hydrogen gas as a reducing atmosphere were examined. The results are shown in FIG. 4, and an enlarged view of the first quadrant is shown as FIG. From FIG. 5, it is understood that the limiting current according to the water vapor concentration can be obtained by applying the monitoring voltage between 0.1 V and 0.6 V by correcting the dry atmosphere as a base. Therefore, the water vapor concentration in the atmosphere to be measured can be known from the limiting current value obtained when the monitoring voltage between 0.1 V and 0.6 V is applied.

[Measurement of Water Vapor Partial Pressure in Non-Oxidizing / Non-Reducing Atmosphere] Meanwhile, the measurement of water vapor partial pressure in an atmosphere that is neither oxidizing nor reducing (hereinafter referred to as "intermediate atmosphere") will be described. FIG. 6 shows the output current when the voltage applied to the sensor element is changed in a nitrogen atmosphere in which the partial pressure of water vapor is changed. It can be seen that when the correction is performed based on the output value for the dry atmosphere, the limiting current value according to the water vapor partial pressure is obtained when the applied voltage is 0.9 V or more. This shows that the partial pressure of water vapor in a non-oxidizing / non-reducing atmosphere can be measured with an applied voltage of 0.9 V to 1.2 V.

As described above, a voltage reverse to that at the time of measurement is applied between the sensor element electrodes to judge whether the atmosphere to be measured is an oxidizing atmosphere, a reducing atmosphere or an intermediate atmosphere, and is suitable for the atmosphere to be measured. Monitoring voltage, for example, 1 V or more, preferably 1.2 V or more in an oxidizing atmosphere, 0.8 V or more and 0.6 V or less in a reducing atmosphere, and 0.9 V or more in an intermediate atmosphere. By using a calibration curve suitable for the atmosphere, the water vapor partial pressure can be measured regardless of the atmosphere to be measured.

[Application to Actual Measurement] FIG. 7 shows a block diagram of the water vapor partial pressure measuring sensor of the present invention. Reference numeral 1 indicates a limiting current type sensor element having a pair of electrodes and applying a monitoring voltage between these electrodes. This water vapor partial pressure measuring sensor is applied between electrodes of the sensor element. Applied voltage control means α for changing the voltage to be applied, current measuring means β for detecting a current flowing between the electrodes, and a current value measured by the current measuring means β and a voltage applied between the electrodes. Atmosphere judging means γ for judging the measurement atmosphere,
The water vapor partial pressure calculating means δ calculates the water vapor partial pressure from the current value detected by the current measuring means by the calibration curve corresponding to the atmosphere detected by the atmosphere determining means γ.

FIG. 8 shows a circuit diagram of an embodiment of the water vapor partial pressure measuring sensor of the present invention. A power source 2 and an ammeter 3 are connected to the sensor element 1. The power source is connected to the output port 4o1 of the MPU 4 and applies a voltage according to the control of the MPU 4 to the electrode of the sensor element. The ammeter converts a current value flowing between the electrodes of the sensor element into a voltage corresponding to the current value, and is usually formed by a known resistor. The voltage value corresponding to this current value is taken into the MPU 4 via the input port 4i having an A / D conversion function. The MPU 4 also has a ROM 4ro for storing control programs, constants, calibration curves, etc., and a RAM for holding data and variables.
4ra, CPU4cp which controls MPU4 by a program and calculates the water vapor partial pressure from the input data, and
An output port O2 is provided for outputting the calculated water vapor partial pressure in order to display it on the LCD 5.

The sensor element temperature is 690 ° C., the monitoring voltage for measuring the partial pressure of water vapor is 0.3 V, the application time is 10 seconds, the monitoring voltage for the redox determination is −0.3 V, and the application time is 2 seconds. FIG. 9 shows a diagram of an example of measuring the partial pressure of water vapor when the above procedure is repeated. In the figure, Vp is the applied voltage, the unit is V, Io is the current value flowing between the electrodes, and S
L indicates a slash level, and when the applied voltage is -0.3 V, it is determined that the oxidizing atmosphere is an oxidizing atmosphere when the current value is less than this value, and when it is larger than this value, the reducing atmosphere is determined. On the other hand, Jo / R is the determination result of the atmosphere to be measured, where 0x represents an oxidizing atmosphere and Re represents a reducing atmosphere. Wv is the moisture content measurement result. In this example, only the reducing atmosphere is measured. On the other hand, V
o is the output for oxygen and is detected only in an oxidizing atmosphere.

In this example, a voltage of -0.3 V is applied for 2 seconds between the electrodes of the sensor element as a redox determination monitoring voltage. If the value of Io 0.2 seconds before the end of this period is less than the slash level, it is determined to be reduction, and 0.3V, which is the monitoring voltage for measuring the partial pressure of water vapor, is applied between the electrodes of the sensor element, It can be seen from FIG. 9 that the output (current) with respect to the partial pressure of water vapor can be obtained at this time.

On the other hand, when the value of Io at the time of the above-mentioned redox determination is above the slash level, it is determined that the atmosphere is in an oxidizing atmosphere, and a voltage of 0.6 V for measuring the oxygen concentration is applied between the sensor electrodes, so that the measured atmosphere It can be seen from FIG. 9 that the oxygen concentration of the is measured. Further, it can be seen from FIG. 9 that the response of the sensor element after switching the monitoring voltage is extremely short, on the order of milliseconds. From this, it is also understood that when the change of the atmosphere frequently occurs in a short cycle, the measurement can be performed with high accuracy by shortening the switching time.

The operation of the water vapor partial pressure measuring sensor according to the present invention has been described above with reference to an example in which the water vapor partial pressure is measured only in the reducing atmosphere and the oxygen concentration is measured in the oxidizing atmosphere. Here, an example will be described in which, after measuring the oxygen concentration in an oxidizing atmosphere, the applied voltage is set to a voltage that can also measure the water vapor partial pressure to measure the water vapor partial pressure in the oxidizing atmosphere. FIG. 10 shows the results of examining the output current when the voltage applied to the sensor element was changed using a sample gas prepared by mixing water vapor with various concentrations of air as a base. From this figure, it can be seen that when the applied voltage exceeds 1 V, the corresponding limiting current of the water vapor concentration is obtained, and when the applied voltage is 1.2 V or more, the sensitivity of the limiting current to the water vapor concentration becomes good. The reason why the output current becomes lower as the water vapor concentration is higher near 0.95 V or lower is that the oxygen concentration becomes relatively lower as the water vapor concentration in the sample gas becomes higher.

Here, the change in the sensor output when the dry air and the steam saturated air at 80 ° C. were alternately switched was examined. The results are shown in Fig. 11. For reference, the result when the monitor voltage is set to 0.8V except when the monitor voltage is 1.2V (after 220 seconds in the figure) (50 seconds to 22 seconds in the figure)
(Around 0 seconds) is also described. "Dry" in the figure
Indicates that the atmosphere of the sensor element was switched to dry air, and “saturated steam” to steam saturated air at 80 ° C., respectively. From FIGS. 10 and 11, it can be seen that the water vapor concentration can be measured even in an oxidizing atmosphere, and the response at that time is extremely good.

[0022]

The method for measuring the partial pressure of water vapor according to the present invention is an excellent method for measuring whether the atmosphere to be measured is an oxidizing atmosphere or a reducing atmosphere. The water vapor partial pressure measuring sensor of the present invention,
Whether the atmosphere to be measured is an oxidizing atmosphere or a reducing atmosphere,
It is an excellent sensor that can be measured, and it can sufficiently cope with frequent changes in the atmosphere.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a limiting current type sensor element used in the present invention.

[Fig. 2] Applied voltage is -0.6V in various atmospheres.
It is a figure which shows the result of having investigated the change of the electric current value at the time of sweeping from the vicinity to the vicinity of 0.6V.

FIG. 3 is a diagram showing a relationship between an applied voltage and an output (current value) with respect to a water vapor concentration in an oxygen-nitrogen mixed gas having an oxygen concentration of 4%.

FIG. 4 is an applied power and an output (current value) with respect to a water vapor concentration in hydrogen gas containing 1% hydrogen gas as a reducing atmosphere.
It is a figure which shows the result of having investigated the influence on.

FIG. 5 is an enlarged view of the first quadrant of FIG.

FIG. 6 is a diagram showing changes in output current when the voltage applied to the sensor element is changed in a nitrogen atmosphere in which the partial pressure of water vapor is changed.

FIG. 7 is a block diagram of a water vapor partial pressure measuring sensor of the present invention.

FIG. 8 is a diagram showing a circuit diagram of an embodiment of a water vapor partial pressure measuring sensor of the present invention.

FIG. 9 is a diagram showing an example of a diagram of a partial vapor pressure measurement example.

FIG. 10 is a diagram showing a relationship between an applied voltage and a sensor output current value in an oxidizing atmosphere having different water vapor concentrations.

FIG. 11 is a diagram showing a change in sensor output when the sensor element atmosphere is switched between dry air and steam saturated air at 80 ° C.

[Explanation of symbols]

α Applied voltage control means β current measuring means γ Measured atmosphere judgment means δ Water vapor partial pressure calculation means

Continuation of front page (56) Reference JP-A-2-276959 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/416 G01N 27/41

Claims (3)

(57) [Claims] 1. A water vapor partial pressure measuring method using a limiting current type sensor element having a pair of electrodes and applying a monitoring voltage between the electrodes, wherein a voltage opposite to that at the time of measurement is applied between the electrodes. Then, the atmosphere to be measured is determined, and a monitoring voltage suitable for the atmosphere to be measured is applied between the electrodes. 2. The water vapor partial pressure measuring method according to claim 1, wherein the atmosphere to be measured is a reducing atmosphere. 3. A water vapor partial pressure measuring sensor using a limiting current type sensor element having a pair of electrodes and applying a monitoring voltage between the electrodes, wherein applied voltage control for changing the voltage applied between the electrodes. Means, current measuring means for detecting a current flowing between the electrodes, and atmosphere determining means for determining the atmosphere to be measured from the current value measured by the current measuring means and the voltage applied between the electrodes, A water vapor partial pressure measuring sensor, comprising: a water vapor partial pressure calculating means for calculating a water vapor partial pressure from a current value detected by the current measuring means by a calibration curve corresponding to the atmosphere detected by the atmosphere determining means.